Fine fiber media layer

ABSTRACT

Disclosed are improved polymer materials. Also disclosed are fine fiber materials that can be made from the improved polymeric materials in the form of microfiber and nanofiber structures. The microfiber and nanofiber structures can be used in a variety of useful applications including the formation of filter materials.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of Ser. No. 13/211,637,filed Aug. 17, 2011, now U.S. Pat. No. 8,366,797, issued Feb. 5, 2013,which is a continuation application of Ser. No. 12/609,567, filed Oct.30, 2009, now U.S. Pat. No. 8,118,901, issued Feb. 21, 2012, which is acontinuation application of Ser. No. 12/008,919, filed Jan. 14, 2008,which was abandoned, which is a continuation application of Ser. No.11/398,788, filed Apr. 6, 2006, now U.S. Pat. No. 7,318,852, issued Jan.18, 2008, which is a continuation of application Ser. No. 10/894,848,filed Jul. 19, 2004, now U.S. Pat. No. 7,179,317, issued Feb. 20, 2007,which is a divisional of application Ser. No. 10/676,189, filed Sep. 30,2003, now U.S. Pat. No. 6,924,028, issued Aug. 2, 2005, whichapplication is a divisional of application Ser. No. 09/871,583, filedMay 31, 2001, now U.S. Pat. No. 6,743,273, issued Jun. 1, 2004, whichclaims the benefit of provisional application Ser. No. 60/230,138, filedSep. 5, 2000, which applications are incorporated herein by reference.

FIELD OF THE INVENTION

The invention also relates to polymer materials can be manufactured withimproved environmental stability to heat, humidity, reactive materialsand mechanical stress. Such materials can be used in the formation offine fibers such as microfibers and nanofiber materials with improvedstability and strength. As the size of fiber is reduced thesurvivability of the materials is increasingly more of a problem. Suchfine fibers are useful in a variety of applications. In one application,filter structures can be prepared using this fine fiber technology. Theinvention relates to polymers, polymeric composition, fiber, filters,filter constructions, and methods of filtering. Applications of theinvention particularly concern filtering of particles from fluidstreams, for example from air streams and liquid (e.g. non-aqueous andaqueous) streams. The techniques described concern structures having oneor more layers of fine fibers in the filter media. The compositions andfiber sizes are selected for a combination of properties andsurvivability.

BACKGROUND OF THE INVENTION

The invention relates to polymeric compositions with improved propertiesthat can be used in a variety of applications including the formation offibers, microfibers, nanofibers, fiber webs, fibrous mats, permeablestructures such as membranes, coatings or films. The polymeric materialsof the invention are compositions that have physical properties thatpermit the polymeric material, in a variety of physical shapes or forms,to have resistance to the degradative effects of humidity, heat, airflow, chemicals and mechanical stress or impact. The invention alsopertains generally to the field of filter-type media for the filtrationof fluids including gaseous and liquid materials. The filter mediaincludes at least a micro- or nanofiber web layer in combination with asubstrate material in a mechanically stable filter structure. Theselayers together provide excellent filtering, high particle capture,efficiency at minimum flow restriction when a fluid such as a gas orliquid passes through the filter media. The substrate can be positionedin the fluid stream upstream, downstream or in an internal layer. Avariety of industries have directed substantial attention in recentyears to the use of filtration media for filtration, i.e. the removal ofunwanted particles from a fluid such as gas or liquid. The commonfiltration process removes particulate from fluids including an airstream or other gaseous stream or from a liquid stream such as ahydraulic fluid, lubricant oil, fuel, water stream or other fluids. Suchfiltration processes require the mechanical strength, chemical andphysical stability of the microfiber and the substrate materials. Thefilter media can be exposed to a broad range of temperature conditions,humidity, mechanical vibration and shock and both reactive andnon-reactive, abrasive or non-abrasive particulates entrained in thefluid flow. Further, the filtration media often require theself-cleaning ability of exposing the filter media to a reverse pressurepulse (a short reversal of fluid flow to remove surface coating ofparticulate) or other cleaning mechanism that can remove entrainedparticulate from the surface of the filter media. Such reverse cleaningcan result in substantially improved (i.e.) reduced pressure drop afterthe pulse cleaning. Particle capture efficiency typically is notimproved after pulse cleaning, however pulse cleaning will reducepressure drop, saving energy for filtration operation. Such filters canbe removed for service and cleaned in aqueous or non-aqueous cleaningcompositions. Such media are often manufactured by spinning fine fiberand then forming an interlocking web of microfiber on a poroussubstrate. In the spinning process the fiber can form physical bondsbetween fibers to interlock the fiber mat into a integrated layer. Sucha material can then be fabricated into the desired filter format such ascartridges, flat disks, canisters, panels, bags and pouches. Within suchstructures, the media can be substantially pleated, rolled or otherwisepositioned on support structures.

In making non-woven fine fiber filter media, a variety of materials havebeen used including fiberglass, metal, ceramics and a wide range ofpolymeric compositions. A variety of techniques have been used for themanufacture of small diameter micro- and nanofibers. One method involvespassing the material through a fine capillary or opening either as amelted material or in a solution that is subsequently evaporated. Fiberscan also be formed by using “spinnerets” typical for the manufacture ofsynthetic fiber such as nylon. Electrostatic spinning is also known.Such techniques involve the use of a hypodermic needle, nozzle,capillary or movable emitter. These structures provide liquid solutionsof the polymer that are then attracted to a collection zone by a highvoltage electrostatic field. As the materials are pulled from theemitter and accelerate through the electrostatic zone, the fiber becomesvery thin and can be formed in a fiber structure by solvent evaporation.

As more demanding applications are envisioned for filtration media,significantly improved materials are required to withstand the rigors ofhigh temperature 100° F. to 250° F. and up to 300° F., high humidity 10%to 90% up to 100% RH, high flow rates of both gas and liquid, andfiltering micron and submicron particulates (ranging from about 0.01 toover 10 microns) and removing both abrasive and non-abrasive andreactive and non-reactive particulate from the fluid stream.

Accordingly, a substantial need exists for polymeric materials, micro-and nanofiber materials and filter structures that provide improvedproperties for filtering streams with higher temperatures, higherhumidities, high flow rates and said micron and submicron particulatematerials.

SUMMARY OF THE INVENTION

The invention provides an improved polymeric material. This polymer hasimproved physical and chemical stability. The polymer fine fiber(microfiber and nanofiber) can be fashioned into useful product formats.Nanofiber is a fiber with diameter less than 200 nanometers or 0.2micron. Microfiber is a fiber with diameter larger than 0.2 micron, butnot larger than 10 microns. This fine fiber can be made in the form ofan improved multi-layer microfiltration media structure. The fine fiberlayers of the invention comprise a random distribution of fine fiberswhich can be bonded to form an interlocking net. Filtration performanceis obtained largely as a result of the fine fiber barrier to the passageof particulate. Structural properties of stiffness, strength,pleatability are provided by the substrate to which the fine fiberadhered. The fine fiber interlocking networks have as importantcharacteristics, fine fibers in the form of microfibers or nanofibersand relatively small spaces between the fibers. Such spaces typicallyrange, between fibers, of about 0.01 to about 25 microns or often about0.1 to about 10 microns. The filter products comprising a fine fiberlayer and a cellulosic layer are thin with a choice of appropriatesubstrate. The fine fiber adds less than a micron in thickness to theoverall fine fiber plus substrate filter media. In service, the filterscan stop incident particulate from passing through the fine fiber layerand can attain substantial surface loadings of trapped particles. Theparticles comprising dust or other incident particulates rapidly form adust cake on the fine fiber surface and maintain high initial andoverall efficiency of particulate removal. Even with relatively finecontaminants having a particle size of about 0.01 to about 1 micron, thefilter media comprising the fine fiber has a very high dust capacity.

The polymer materials as disclosed herein have substantially improvedresistance to the undesirable effects of heat, humidity, high flowrates, reverse pulse cleaning, operational abrasion, submicronparticulates, cleaning of filters in use and other demanding conditions.The improved microfiber and nanofiber performance is a result of theimproved character of the polymeric materials forming the microfiber ornanofiber. Further, the filter media of the invention using the improvedpolymeric materials of the invention provides a number of advantageousfeatures including higher efficiency, lower flow restriction, highdurability (stress related or environmentally related) in the presenceof abrasive particulates and a smooth outer surface free of loose fibersor fibrils. The overall structure of the filter materials provides anoverall thinner media allowing improved media area per unit volume,reduced velocity through the media, improved media efficiency andreduced flow restrictions.

A preferred mode of the invention is a polymer blend comprising a firstpolymer and a second, but different polymer (differing in polymer type,molecular weight or physical property) that is conditioned or treated atelevated temperature. The polymer blend can be reacted and formed into asingle chemical specie or can be physically combined into a blendedcomposition by an annealing process. Annealing implies a physicalchange, like crystallinity, stress relaxation or orientation. Preferredmaterials are chemically reacted into a single polymeric specie suchthat a Differential Scanning calorimeter analysis reveals a singlepolymeric material. Such a material, when combined with a preferredadditive material, can form a surface coating of the additive on themicrofiber that provides oleophobicity, hydrophobicity or otherassociated improved stability when contacted with high temperature, highhumidity and difficult operating conditions. The fine fiber of the classof materials can have a diameter of 2 microns to less than 0.01 micron.Such microfibers can have a smooth surface comprising a discrete layerof the additive material or an outer coating of the additive materialthat is partly solubilized or alloyed in the polymer surface, or both.Preferred materials for use in the blended polymeric systems includenylon 6; nylon 66; nylon 6-10; nylon (6-66-610) copolymers and otherlinear generally aliphatic nylon compositions. A preferred nyloncopolymer resin (SVP-651) was analyzed for molecular weight by the endgroup titration. (J. E. Walz and G. B. Taylor, determination of themolecular weight of nylon, Anal. Chem. Vol. 19, Number 7, pp 448-450(1947). A number average molecular weight (W_(n)) was between 21,500 and24,800. The composition was estimated by the phase diagram of melttemperature of three component nylon, nylon 6 about 45%, nylon 66 about20% and nylon 610 about 25%. (Page 286, Nylon Plastics Handbook, MelvinKohan ed. Hanser Publisher, New York (1995)).

Reported physical properties of SVP 651 resin are:

Property ASTM Method Units Typical Value Specific Gravity D-792 —   1.08 Water Absorption D-570 %   2.5 (24 hr immersion) Hardness D-240Shore D 65 Melting Point DSC ° C. (° F.) 154 (309) Tensile StrengthD-638 MPa (kpsi)  50 (7.3) @ Yield Elongation at Break D-638 % 350 Flexural Modulus D-790 MPa (kpsi) 180 (26)  Volume Resistivity D-257ohm-cm  10¹²

A polyvinylalcohol having a hydrolysis degree of from 87 to 99.9+% canbe used in such polymer systems. These are preferably cross linked. Andthey are most preferably crosslinked and combined with substantialquantities of the oleophobic and hydrophobic additive materials.

Another preferred mode of the invention involves a single polymericmaterial combined with an additive composition to improve fiber lifetimeor operational properties. The preferred polymers useful in this aspectof the invention include nylon polymers, polyvinylidene chloridepolymers, polyvinylidene fluoride polymers, polyvinylalcohol polymersand, in particular, those listed materials when combined with stronglyoleophobic and hydrophobic additives that can result in a microfiber ornanofiber with the additive materials formed in a coating on the finefiber surface. Again, blends of similar polymers such as a blend ofsimilar nylons, similar polyvinylchloride polymers, blends ofpolyvinylidene chloride polymers are useful in this invention. Further,polymeric blends or alloys of differing polymers are also contemplatedby the invention. In this regard, compatible mixtures of polymers areuseful in forming the microfiber materials of the invention. Additivecomposition such a fluoro-surfactant, a nonionic surfactant, lowmolecular weight resins (e.g.) tertiary butylphenol resin having amolecular weight of less than about 3000 can be used. The resin ischaracterized by oligomeric bonding between phenol nuclei in the absenceof methylene bridging groups. The positions of the hydroxyl and thetertiary butyl group can be randomly positioned around the rings.Bonding between phenolic nuclei always occurs next to hydroxyl group,not randomly. Similarly, the polymeric material can be combined with analcohol soluble non-linear polymerized resin formed from bis-phenol A.Such material is similar to the tertiary butylphenol resin describedabove in that it is formed using oligomeric bonds that directly connectaromatic ring to aromatic ring in the absence of any bridging groupssuch as alkylene or methylene groups.

A particularly preferred material of the invention comprises amicrofiber material having a dimension of about 2 to 0.01 microns. Themost preferred fiber size range between 0.05 to 0.5 micron. Such fiberswith the preferred size provide excellent filter activity, ease of backpulse cleaning and other aspects. The highly preferred polymer systemsof the invention have adhering characteristic such that when contactedwith a cellulosic substrate adheres to the substrate with sufficientstrength such that it is securely bonded to the substrate and can resistthe delaminating effects of a reverse pulse cleaning technique and othermechanical stresses. In such a mode, the polymer material must stayattached to the substrate while undergoing a pulse clean input that issubstantially equal to the typical filtration conditions except in areverse direction across the filter structure. Such adhesion can arisefrom solvent effects of fiber formation as the fiber is contacted withthe substrate or the post treatment of the fiber on the substrate withheat or pressure. However, polymer characteristics appear to play animportant role in determining adhesion, such as specific chemicalinteractions like hydrogen bonding, contact between polymer andsubstrate occurring above or below Tg, and the polymer formulationincluding additives. Polymers plasticized with solvent or steam at thetime of adhesion can have increased adhesion.

An important aspect of the invention is the utility of such microfiberor nanofiber materials formed into a filter structure. In such astructure, the fine fiber materials of the invention are formed on andadhered to a filter substrate. Natural fiber and synthetic fibersubstrates, like spunbonded fabrics, non-woven fabrics of syntheticfiber and non-wovens made from the blends of cellulosics, synthetic andglass fibers, non-woven and woven glass fabrics, plastic screen likematerials both extruded and hole punched, UF and MF membranes of organicpolymers can be used. Sheet-like substrate or cellulosic non-woven webcan then be formed into a filter structure that is placed in a fluidstream including an air stream or liquid stream for the purpose ofremoving suspended or entrained particulate from that stream. The shapeand structure of the filter material is up to the design engineer. Oneimportant parameter of the filter elements after formation is itsresistance to the effects of heat, humidity or both. One aspect of thefilter media of the invention is a test of the ability of the filtermedia to survive immersion in warm water for a significant period oftime. The immersion test can provide valuable information regarding theability of the fine fiber to survive hot humid conditions and to survivethe cleaning of the filter element in aqueous solutions that can containsubstantial proportions of strong cleaning surfactants and strongalkalinity materials. Preferably, the fine fiber materials of theinvention can survive immersion in hot water while retaining at least50% of the fine fiber formed on the surface of the substrate. Retentionof at least 50% of the fine fiber can maintain substantial fiberefficiency without loss of filtration capacity or increased backpressure. Most preferably retaining at least 75%.

A fine fiber filter structure includes a bi-layer or multi-layerstructure wherein the filter contains one or more fine fiber layerscombined with or separated by one or more synthetic, cellulosic orblended webs. Another preferred motif is a structure including finefiber in a matrix or blend of other fibers.

We believe important characteristics of the fiber and microfiber layersin the filter structure relate to temperature resistance, humidity ormoisture resistance and solvent resistance, particularly when themicrofiber is contacted with humidity, moisture or a solvent at elevatedtemperatures. Further, a second important property of the materials ofthe invention relates to the adhesion of the material to a substratestructure. The microfiber layer adhesion is an important characteristicof the filter material such that the material can be manufacturedwithout delaminating the microfiber layer from the substrate, themicrofiber layer plus substrate can be processed into a filter structureincluding pleats, rolled materials and other structures withoutsignificant delamination. We have found that the heating step of themanufacturing process wherein the temperature is raised to a temperatureat or near but just below melt temperature of one polymer material,typically lower than the lowest melt temperature substantially improvesthe adhesion of the fibers to each other and the substrate. At or abovethe melt temperature, the fine fiber can lose its fibrous structure. Itis also critical to control heating rate. If the fiber is exposed to itscrystallization temperature for extended period of time, it is alsopossible to lose fibrous structure. Careful heat treatment also improvedpolymer properties that result from the formation of the exterioradditive layers as additive materials migrate to the surface and exposehydrophobic or oleophobic groups on the fiber surface.

The criteria for performance is that the material be capable ofsurviving intact various operating temperatures, i.e. a temperature of140° F., 160° F., 270° F., 300° F. for a period of time of 1 hour or 3hours, depending on end use, while retaining 30%, 50%, 80% or 90% offilter efficiency. An alternative criteria for performances that thematerial is capable of surviving intact at various operatingtemperatures, i.e. temperatures of 140° F., 160° F., 270° F., 300° F.,for a period of time of 1 hours or 3 hours depending on end use, whileretaining, depending on end use, 30%, 50%, 80% or 90% of effective finefibers in a filter layer. Survival at these temperatures is important atlow humidity, high humidity, and in water saturated air. The microfiberand filter material of the invention are deemed moisture resistant wherethe material can survive immersion at a temperature of greater than 160°F. while maintaining efficiency for a time greater than about 5 minutes.Similarly, solvent resistance in the microfiber material and the filtermaterial of the invention is obtained from a material that can survivecontact with a solvent such as ethanol, a hydrocarbon, a hydraulicfluid, or an aromatic solvent for a period of time greater than about 5minutes at 70° F. while maintaining 50% efficiency.

The fine fiber materials of the invention can be used in a variety offilter applications including pulse clean and non-pulse cleaned filtersfor dust collection, gas turbines and engine air intake or inductionsystems; gas turbine intake or induction systems, heavy duty engineintake or induction systems, light vehicle engine intake or inductionsystems; Z filter; vehicle cabin air; off road vehicle cabin air, diskdrive air, photocopier-toner removal; HVAC filters in both commercial orresidential filtration applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a typical electrostatic emitter driven apparatus forproduction of the fine fibers of the invention.

FIG. 2 shows the apparatus used to introduce fine fiber onto filtersubstrate into the fine fiber forming technology shown in FIG. 1.

FIG. 3 is a depiction of the typical internal structure of a supportmaterial and a separate depiction of the fine fiber material of theinvention compared to small, i.e. 2 and 5 micron particulate materials.

FIGS. 4 through 11 are analytical ESCA spectra relating to Example 13.

FIG. 12 shows the stability of the 0.23 and 0.45 microfiber material ofthe invention from Example 5.

FIGS. 13 through 16 show the improved temperature and humidity stabilityof the materials of Examples 5 and 6 when compared to unmodified nyloncopolymer solvent soluble polyamide.

FIGS. 17 through 20 demonstrate that the blend of two copolymers, anylon homopolymer and a nylon copolymer, once heat treated and combinedwith additives form a single component material that does not displaydistinguishable characteristics of two separate polymer materials, butappears to be a crosslinked or otherwise chemically joined single phase.

DETAILED DESCRIPTION OF THE INVENTION

The fine fibers that comprise the micro- or nanofiber containing layerof the invention can be fiber and can have a diameter of about 0.001 to2 micron, preferably 0.05 to 0.5 micron. The thickness of the typicalfine fiber filtration layer ranges from about 1 to 100 times the fiberdiameter with a basis weight ranging from about 0.01 to 240micrograms-cm⁻².

Fluid streams such as air and gas streams often carry particulatematerial therein. The removal of some or all of the particulate materialfrom the fluid stream is needed. For example, air intake streams to thecabins of motorized vehicles, air in computer disk drives, HVAC air,clean room ventilation and applications using filter bags, barrierfabrics, woven materials, air to engines for motorized vehicles, or topower generation equipment; gas streams directed to gas turbines; and,air streams to various combustion furnaces, often include particulatematerial therein. In the case of cabin air filters it is desirable toremove the particulate matter for comfort of the passengers and/or foraesthetics. With respect to air and gas intake streams to engines, gasturbines and combustion furnaces, it is desirable to remove theparticulate material because particulate can cause substantial damage tothe internal workings to the various mechanisms involved. In otherinstances, production gases or off gases from industrial processes orengines may contain particulate material therein. Before such gases canbe, or should be, discharged through various downstream equipment to theatmosphere, it may be desirable to obtain a substantial removal ofparticulate material from those streams.

A general understanding of some of the basic principles and problems ofair filter design can be understood by consideration of the followingtypes of filter media: surface loading media; and, depth media. Each ofthese types of media has been well studied, and each has been widelyutilized. Certain principles relating to them are described, forexample, in U.S. Pat. Nos. 5,082,476; 5,238,474; and 5,364,456. Thecomplete disclosures of these three patents are incorporated herein byreference.

The “lifetime” of a filter is typically defined according to a selectedlimiting pressure drop across the filter. The pressure buildup acrossthe filter defines the lifetime at a defined level for that applicationor design. Since this buildup of pressure is a result of load, forsystems of equal efficiency a longer life is typically directlyassociated with higher capacity. Efficiency is the propensity of themedia to trap, rather than pass, particulates. It should be apparentthat typically the more efficient a filter media is at removingparticulates from a gas flow stream, in general the more rapidly thefilter media will approach the “lifetime” pressure differential(assuming other variables to be held constant). In this application theterm “unchanged for filtration purposes” refers to maintainingsufficient efficiency to remove particulate from the fluid stream as isnecessary for the selected application.

Paper filter elements are widely used forms of surface loading media. Ingeneral, paper elements comprise dense mats of cellulose, synthetic orother fibers oriented across a gas stream carrying particulate material.The paper is generally constructed to be permeable to the gas flow, andto also have a sufficiently fine pore size and appropriate porosity toinhibit the passage of particles greater than a selected sizetherethrough. As the gases (fluids) pass through the filter paper, theupstream side of the filter paper operates through diffusion andinterception to capture and retain selected sized particles from the gas(fluid) stream. The particles are collected as a dust cake on theupstream side of the filter paper. In time, the dust cake also begins tooperate as a filter, increasing efficiency. This is sometimes referredto as “seasoning,” i.e. development of an efficiency greater thaninitial efficiency.

A simple filter design such as that described above is subject to atleast two types of problems. First, a relatively simple flaw, i.e.rupture of the paper, results in failure of the system. Secondly,particulate material rapidly builds up on the upstream side of thefilter, as a thin dust cake or layer, increasing the pressure drop.Various methods have been applied to increase the “lifetime” ofsurface-loaded filter systems, such as paper filters. One method is toprovide the media in a pleated construction, so that the surface area ofmedia encountered by the gas flow stream is increased relative to aflat, non-pleated construction. While this increases filter lifetime, itis still substantially limited. For this reason, surface loaded mediahas primarily found use in applications wherein relatively lowvelocities through the filter media are involved, generally not higherthan about 20-30 feet per minute and typically on the order of about 10feet per minute or less. The term “velocity” in this context is theaverage velocity through the media (i.e. flow volume per media area).

In general, as air flow velocity is increased through a pleated papermedia, filter life is decreased by a factor proportional to the squareof the velocity. Thus, when a pleated paper, surface loaded, filtersystem is used as a particulate filter for a system that requiressubstantial flows of air, a relatively large surface area for the filtermedia is needed. For example, a typical cylindrical pleated paper filterelement of an over-the-highway diesel truck will be about 9-15 inches indiameter and about 12-24 inches long, with pleats about 1-2 inches deep.Thus, the filtering surface area of media (one side) is typically 30 to300 square feet.

In many applications, especially those involving relatively high flowrates, an alternative type of filter media, sometimes generally referredto as “depth” media, is used. A typical depth media comprises arelatively thick tangle of fibrous material. Depth media is generallydefined in terms of its porosity, density or percent solids content. Forexample, a 2-3% solidity media would be a depth media mat of fibersarranged such that approximately 2-3% of the overall volume comprisesfibrous materials (solids), the remainder being air or gas space.

Another useful parameter for defining depth media is fiber diameter. Ifpercent solidity is held constant, but fiber diameter (size) is reduced,pore size or interfiber space is reduced, i.e. the filter becomes moreefficient and will more effectively trap smaller particles.

A typical conventional depth media filter is a deep, relatively constant(or uniform) density, media, i.e. a system in which the solidity of thedepth media remains substantially constant throughout its thickness. By“substantially constant” in this context, it is meant that onlyrelatively minor fluctuations in density, if any, are found throughoutthe depth of the media. Such fluctuations, for example, may result froma slight compression of an outer engaged surface, by a container inwhich the filter media is positioned.

Gradient density depth media arrangements have been developed. Some sucharrangements are described, for example, in U.S. Pat. Nos. 4,082,476;5,238,474; and 5,364,456. In general, a depth media arrangement can bedesigned to provide “loading” of particulate materials substantiallythroughout its volume or depth. Thus, such arrangements can be designedto load with a higher amount of particulate material, relative tosurface loaded systems, when full filter lifetime is reached. However,in general the tradeoff for such arrangements has been efficiency,since, for substantial loading, a relatively low solidity media isdesired. Gradient density systems such as those in the patents referredto above, have been designed to provide for substantial efficiency andlonger life. In some instances, surface loading media is utilized as a“polish” filter in such arrangements.

Polymeric materials have been fabricated in non-woven and woven fabrics,fibers and microfibers. The polymeric material provides the physicalproperties required for product stability. These materials should notchange significantly in dimension, suffer reduced molecular weight,become less flexible or subject to stress cracking or physicallydeteriorate in the presence of sunlight, humidity, high temperatures orother negative environmental effects. The invention relates to animproved polymeric material that can maintain physical properties in theface of incident electromagnetic radiation such as environmental light,heat, humidity and other physical challenges.

Polymer materials that can be used in the polymeric compositions of theinvention include both addition polymer and condensation polymermaterials such as polyolefin, polyacetal, polyamide, polyester,cellulose ether and ester, polyalkylene sulfide, polyarylene oxide,polysulfone, modified polysulfone polymers and mixtures thereof.Preferred materials that fall within these generic classes includepolyethylene, polypropylene, poly(vinylchloride), polymethylmethacrylate(and other acrylic resins), polystyrene, and copolymers thereof(including ABA type block copolymers), poly(vinylidene fluoride),poly(vinylidene chloride), polyvinylalcohol in various degrees ofhydrolysis (87% to 99.5%) in crosslinked and non-crosslinked forms.Preferred addition polymers tend to be glassy (a Tg greater than roomtemperature). This is the case for polyvinylchloride andpolymethylmethacrylate, polystyrene polymer compositions or alloys orlow in crystallinity for polyvinylidene fluoride and polyvinylalcoholmaterials. One class of polyamide condensation polymers are nylonmaterials. The term “nylon” is a generic name for all long chainsynthetic polyamides. Typically, nylon nomenclature includes a series ofnumbers such as in nylon-6,6 which indicates that the starting materialsare a C₆ diamine and a C₆ diacid (the first digit indicating a C₆diamine and the second digit indicating a C₆ dicarboxylic acidcompound). Another nylon can be made by the polycondensation of epsiloncaprolactam in the presence of a small amount of water. This reactionforms a nylon-6 (made from a cyclic lactam—also known asepisilon-aminocaproic acid) that is a linear polyamide. Further, nyloncopolymers are also contemplated. Copolymers can be made by combiningvarious diamine compounds, various diacid compounds and various cycliclactam structures in a reaction mixture and then forming the nylon withrandomly positioned monomeric materials in a polyamide structure. Forexample, a nylon 6,6-6,10 material is a nylon manufactured fromhexamethylene diamine and a C₆ and a C₁₀ blend of diacids. A nylon 6-6,6-6,10 is a nylon manufactured by copolymerization ofepsilonaminocaproic acid, hexamethylene diamine and a blend of a C₆ anda C₁₀ diacid material.

Block copolymers are also useful in the process of this invention. Withsuch copolymers the choice of solvent swelling agent is important. Theselected solvent is such that both blocks were soluble in the solvent.One example is an ABA (styrene-EP-styrene) or AB (styrene-EP) polymer inmethylene chloride solvent. If one component is not soluble in thesolvent, it will form a gel. Examples of such block copolymers areKraton® type of styrene-b-butadiene and styrene-b-hydrogenated butadiene(ethylene propylene), Pebax® type of ε-caprolactam-b-ethylene oxide,Sympatex® polyester-b-ethylene oxide and polyurethanes of ethylene oxideand isocyanates.

Addition polymers like polyvinylidene fluoride, syndiotacticpolystyrene, copolymer of vinylidene fluoride and hexafluoropropylene,polyvinyl alcohol, polyvinyl acetate, amorphous addition polymers, suchas poly(acrylonitrile) and its copolymers with acrylic acid andmethacrylates, polystyrene, poly(vinyl chloride) and its variouscopolymers, poly(methyl methacrylate) and its various copolymers, can besolution spun with relative ease because they are soluble at lowpressures and temperatures.

However, highly crystalline polymer like polyethylene and polypropylenerequire high temperature, high pressure solvent if they are to besolution spun. Therefore, solution spinning of the polyethylene andpolypropylene is very difficult. Electrostatic solution spinning is onemethod of making nanofibers and microfiber.

We have also found a substantial advantage to forming polymericcompositions comprising two or more polymeric materials in polymeradmixture, alloy format or in a crosslinked chemically bonded structure.We believe such polymer compositions improve physical properties bychanging polymer attributes such as improving polymer chain flexibilityor chain mobility, increasing overall molecular weight and providingreinforcement through the formation of networks of polymeric materials.

In one embodiment of this concept, two related polymer materials can beblended for beneficial properties. For example, a high molecular weightpolyvinylchloride can be blended with a low molecular weightpolyvinylchloride. Similarly, a high molecular weight nylon material canbe blended with a low molecular weight nylon material. Further,differing species of a general polymeric genus can be blended. Forexample, a high molecular weight styrene material can be blended with alow molecular weight, high impact polystyrene. A Nylon-6 material can beblended with a nylon copolymer such as a Nylon-6; 6,6; 6,10 copolymer.Further, a polyvinylalcohol having a low degree of hydrolysis such as a87% hydrolyzed polyvinylalcohol can be blended with a fully orsuperhydrolyzed polyvinylalcohol having a degree of hydrolysis between98 and 99.9% and higher. All of these materials in admixture can becrosslinked using appropriate crosslinking mechanisms. Nylons can becrosslinked using crosslinking agents that are reactive with thenitrogen atom in the amide linkage. Polyvinylalcohol materials can becrosslinked using hydroxyl reactive materials such as monoaldehydes,such as formaldehyde, ureas, melamine-formaldehyde resin and itsanalogues, boric acids and other inorganic compounds. dialdehydes,diacids, urethanes, epoxies and other known crosslinking agents.Crosslinking technology is a well known and understood phenomenon inwhich a crosslinking reagent reacts and forms covalent bonds betweenpolymer chains to substantially improve molecular weight, chemicalresistance, overall strength and resistance to mechanical degradation.

We have found that additive materials can significantly improve theproperties of the polymer materials in the form of a fine fiber. Theresistance to the effects of heat, humidity, impact, mechanical stressand other negative environmental effect can be substantially improved bythe presence of additive materials. We have found that while processingthe microfiber materials of the invention, that the additive materialscan improve the oleophobic character, the hydrophobic character and canappear to aid in improving the chemical stability of the materials. Webelieve that the fine fibers of the invention in the form of amicrofiber are improved by the presence of these oleophobic andhydrophobic additives as these additives form a protective layercoating, ablative surface or penetrate the surface to some depth toimprove the nature of the polymeric material. We believe the importantcharacteristics of these materials are the presence of a stronglyhydrophobic group that can preferably also have oleophobic character.Strongly hydrophobic groups include fluorocarbon groups, hydrophobichydrocarbon surfactants or blocks and substantially hydrocarbonoligomeric compositions. These materials are manufactured incompositions that have a portion of the molecule that tends to becompatible with the polymer material affording typically a physical bondor association with the polymer while the strongly hydrophobic oroleophobic group, as a result of the association of the additive withthe polymer, forms a protective surface layer that resides on thesurface or becomes alloyed with or mixed with the polymer surfacelayers. For 0.2-micron fiber with 10% additive level, the surfacethickness is calculated to be around 50 Å, if the additive has migratedtoward the surface. Migration is believed to occur due to theincompatible nature of the oleophobic or hydrophobic groups in the bulkmaterial. A 50 Å thickness appears to be reasonable thickness forprotective coating. For 0.05-micron diameter fiber, 50 Å thicknesscorresponds to 20% mass. For 2 microns thickness fiber, 50 Å thicknesscorresponds to 2% mass. Preferably the additive materials are used at anamount of about 2 to 25 wt. %. Oligomeric additives that can be used incombination with the polymer materials of the invention includeoligomers having a molecular weight of about 500 to about 5000,preferably about 500 to about 3000 including fluoro-chemicals, nonionicsurfactants and low molecular weight resins or oligomers. Fluoro-organicwetting agents useful in this invention are organic moleculesrepresented by the formulaR_(f)-Gwherein R_(f) is a fluoroaliphatic radical and G is a group whichcontains at least one hydrophilic group such as cationic, anionic,nonionic, or amphoteric groups. Nonionic materials are preferred. R_(f)is a fluorinated, monovalent, aliphatic organic radical containing atleast two carbon atoms. Preferably, it is a saturated perfluoroaliphaticmonovalent organic radical. However, hydrogen or chlorine atoms can bepresent as substituents on the skeletal chain. While radicals containinga large number of carbon atoms may function adequately, compoundscontaining not more than about 20 carbon atoms are preferred since largeradicals usually represent a less efficient utilization of fluorine thanis possible with shorter skeletal chains. Preferably, R_(f) containsabout 2 to 8 carbon atoms.

The cationic groups that are usable in the fluoro-organic agentsemployed in this invention may include an amine or a quaternary ammoniumcationic group which can be oxygen-free (e.g., —NH₂) oroxygen-containing (e.g., amine oxides). Such amine and quaternaryammonium cationic hydrophilic groups can have formulas such as —NH₂,—(NH₃)X, —(NH(R²)₂)X, —(NH(R²)₃)X, or —N(R₂)₂→O, where x is an anioniccounterion such as halide, hydroxide, sulfate, bisulfate, orcarboxylate, R² is H or C₁₋₁₈ alkyl group, and each R² can be the sameas or different from other R² groups. Preferably, R² is H or a C₁₋₁₆alkyl group and X is halide, hydroxide, or bisulfate.

The anionic groups which are usable in the fluoro-organic wetting agentsemployed in this invention include groups which by ionization can becomeradicals of anions. The anionic groups may have formulas such as —COOM,—SO₃M, —OSO₃M, —PO₃HM, —OPO₃M₂, or —OPO₃HM, where M is H, a metal ion,(NR¹ ₄)⁺, or (SR¹ ₄)⁺, where each R¹ is independently H or substitutedor unsubstituted C₁-C₆ alkyl. Preferably M is Na⁺ or K⁺. The preferredanionic groups of the fluoro-organo wetting agents used in thisinvention have the formula —COOM or —SO₃M. Included within the group ofanionic fluoro-organic wetting agents are anionic polymeric materialstypically manufactured from ethylenically unsaturated carboxylic mono-and diacid monomers having pendent fluorocarbon groups appended thereto.Such materials include surfactants obtained from 3M Corporation known asFC-430 and FC-431.

The amphoteric groups which are usable in the fluoro-organic wettingagent employed in this invention include groups which contain at leastone cationic group as defined above and at least one anionic group asdefined above.

The nonionic groups which are usable in the fluoro-organic wettingagents employed in this invention include groups which are hydrophilicbut which under pH conditions of normal agronomic use are not ionized.The nonionic groups may have formulas such as —O(CH₂CH₂)_(x)OH where xis greater than 1, —SO₂NH₂, —SO₂NHCH₂CH₂OH, —SO₂N(CH₂CH₂H)₂, —CONH₂,—CONHCH₂CH₂OH, or —CON(CH₂CH₂OH)₂. Examples of such materials includematerials of the following structure:F(CF₂CF₂)_(n)—CH₂CH₂O—(CH₂CH₂O)_(m)—Hwherein n is 2 to 8 and m is 0 to 20.

Other fluoro-organic wetting agents include those cationicfluorochemicals described, for example in U.S. Pat. Nos. 2,764,602;2,764,603; 3,147,064 and 4,069,158. Such amphoteric fluoro-organicwetting agents include those amphoteric fluorochemicals described, forexample, in U.S. Pat. Nos. 2,764,602; 4,042,522; 4,069,158; 4,069,244;4,090,967; 4,161,590 and 4,161,602. Such anionic fluoro-organic wettingagents include those anionic fluorochemicals described, for example, inU.S. Pat. Nos. 2,803,656; 3,255,131; 3,450,755 and 4,090,967.

Examples of such materials are duPont Zonyl FSN and duPont Zonyl FSOnonionic surfactants. Another aspect of additives that can be used inthe polymers of the invention include low molecular weight fluorocarbonacrylate materials such as 3M's Scotchgard® material having the generalstructure:CF₃(CX₂)_(n)-acrylatewherein X is —F or —CF₃ and n is 1 to 7.

Further, nonionic hydrocarbon surfactants including lower alcoholethoxylates, fatty acid ethoxylates, nonylphenol ethoxylates, etc. canalso be used as additive materials for the invention. Examples of thesematerials include Triton X-100 and Triton N-101.

A useful material for use as an additive material in the compositions ofthe invention are tertiary butylphenol oligomers. Such materials tend tobe relatively low molecular weight aromatic phenolic resins. Such resinsare phenolic polymers prepared by enzymatic oxidative coupling. Theabsence of methylene bridges result in unique chemical and physicalstability. These phenolic resins can be crosslinked with various aminesand epoxies and are compatible with a variety of polymer materials.These materials are generally exemplified by the following structuralformulas which are characterized by phenolic materials in a repeatingmotif in the absence of methylene bridge groups having phenolic andaromatic groups.

wherein n is 2 to 20. Examples of these phenolic materials includeEnzo-BPA, Enzo-BPA/phenol, Enzo-TBP, Enzo-COP and other relatedphenolics were obtained from Enzymol International Inc., Columbus, Ohio.

It should be understood that an extremely wide variety of fibrous filtermedia exist for different applications. The durable nanofibers andmicrofibers described in this invention can be added to any of themedia. The fibers described in this invention can also be used tosubstitute for fiber components of these existing media giving thesignificant advantage of improved performance (improved efficiencyand/or reduced pressure drop) due to their small diameter, whileexhibiting greater durability.

Polymer nanofibers and microfibers are known, however their use has beenvery limited due to their fragility to mechanical stresses, and theirsusceptibility to chemical degradation due to their very high surfacearea to volume ratio. The fibers described in this invention addressthese limitations and will therefore be usable in a very wide variety offiltration, textile, membrane and other diverse applications.

A filter media construction according to the present invention includesa first layer of permeable coarse fibrous media or substrate having afirst surface. A first layer of fine fiber media is secured to the firstsurface of the first layer of permeable coarse fibrous media. Preferablythe first layer of permeable coarse fibrous material comprises fibershaving an average diameter of at least 10 microns, typically andpreferably about 12 (or 14) to 30 microns. Also preferably the firstlayer of permeable coarse fibrous material comprises a media having abasis weight of no greater than about 200 grams/meter², preferably about0.50 to 150 g/m², and most preferably at least 8 g/m². Preferably thefirst layer of permeable coarse fibrous media is at least 0.0005 inch(12 microns) thick, and typically and preferably is about 0.001 to 0.030inch (25-800 microns) thick.

In preferred arrangements, the first layer of permeable coarse fibrousmaterial comprises a material which, if evaluated separately from aremainder of the construction by the Frazier permeability test, wouldexhibit a permeability of at least 1 meter(s)/min, and typically andpreferably about 2-900 meters/min. Herein when reference is made toefficiency, unless otherwise specified, reference is meant to efficiencywhen measured according to ASTM-1215-89, with 0.78μ monodispersepolystyrene spherical particles, at 20 fpm (6.1 meters/min) as describedherein.

Preferably the layer of fine fiber material secured to the first surfaceof the layer of permeable coarse fibrous media is a layer of nano- andmicrofiber media wherein the fibers have average fiber diameters of nogreater than about 2 microns, generally and preferably no greater thanabout 1 micron, and typically and preferably have fiber diameterssmaller than 0.5 micron and within the range of about 0.05 to 0.5micron. Also, preferably the first layer of fine fiber material securedto the first surface of the first layer of permeable coarse fibrousmaterial has an overall thickness that is no greater than about 30microns, more preferably no more than 20 microns, most preferably nogreater than about 10 microns, and typically and preferably that iswithin a thickness of about 1-8 times (and more preferably no more than5 times) the fine fiber average diameter of the layer.

Certain preferred arrangements according to the present inventioninclude filter media as generally defined, in an overall filterconstruction. Some preferred arrangements for such use comprise themedia arranged in a cylindrical, pleated configuration with the pleatsextending generally longitudinally, i.e. in the same direction as alongitudinal axis of the cylindrical pattern. For such arrangements, themedia may be imbedded in end caps, as with conventional filters. Sucharrangements may include upstream liners and downstream liners ifdesired, for typical conventional purposes.

In some applications, media according to the present invention may beused in conjunction with other types of media, for example conventionalmedia, to improve overall filtering performance or lifetime. Forexample, media according to the present invention may be laminated toconventional media, be utilized in stack arrangements; or beincorporated (an integral feature) into media structures including oneor more regions of conventional media. It may be used upstream of suchmedia, for good load; and/or, it may be used downstream fromconventional media, as a high efficiency polishing filter.

Certain arrangements according to the present invention may also beutilized in liquid filter systems, i.e. wherein the particulate materialto be filtered is carried in a liquid. Also, certain arrangementsaccording to the present invention may be used in mist collectors, forexample arrangements for filtering fine mists from air.

According to the present invention, methods are provided for filtering.The methods generally involve utilization of media as described toadvantage, for filtering. As will be seen from the descriptions andexamples below, media according to the present invention can bespecifically configured and constructed to provide relatively long lifein relatively efficient systems, to advantage.

Various filter designs are shown in patents disclosing and claimingvarious aspects of filter structure and structures used with the filtermaterials. Engel et al., U.S. Pat. No. 4,720,292, disclose a radial sealdesign for a filter assembly having a generally cylindrical filterelement design, the filter element being sealed by a relatively soft,rubber-like end cap having a cylindrical, radially inwardly facingsurface. Kahlbaugh et al., U.S. Pat. No. 5,082,476, disclose a filterdesign using a depth media comprising a foam substrate with pleatedcomponents combined with the microfiber materials of the invention.Stifelman et al., U.S. Pat. No. 5,104,537, relate to a filter structureuseful for filtering liquid media. Liquid is entrained into the filterhousing, passes through the exterior of the filter into an interiorannular core and then returns to active use in the structure. Suchfilters are highly useful for filtering hydraulic fluids. Engel et al.,U.S. Pat. No. 5,613,992, show a typical diesel engine air intake filterstructure. The structure obtains air from the external aspect of thehousing that may or may not contain entrained moisture. The air passesthrough the filter while the moisture can pass to the bottom of thehousing and can drain from the housing. Gillingham et al., U.S. Pat. No.5,820,646, disclose a Z filter structure that uses a specific pleatedfilter design involving plugged passages that require a fluid stream topass through at least one layer of filter media in a “Z” shaped path toobtain proper filtering performance. The filter media formed into thepleated Z shaped format can contain the fine fiber media of theinvention. Glen et al., U.S. Pat. No. 5,853,442, disclose a bag housestructure having filter elements that can contain the fine fiberstructures of the invention. Berkhoel et al., U.S. Pat. No. 5,954,849,show a dust collector structure useful in processing typically airhaving large dust loads to filter dust from an air stream afterprocessing a workpiece generates a significant dust load in anenvironmental air. Lastly, Gillingham, U.S. Design Pat. No. 425,189,discloses a panel filter using the Z filter design.

DETAILED DESCRIPTION OF CERTAIN DRAWINGS

The microfiber or nanofiber of the unit can be formed by theelectrostatic spinning process. A suitable apparatus for forming thefiber is illustrated in FIG. 1. This apparatus includes a reservoir 80in which the fine fiber forming polymer solution is contained, a pump 81and a rotary type emitting device or emitter 40 to which the polymericsolution is pumped. The emitter 40 generally consists of a rotatingunion 41, a rotating portion 42 including a plurality of offset holes 44and a shaft 43 connecting the forward facing portion and the rotatingunion. The rotating union 41 provides for introduction of the polymersolution to the forward facing portion 42 through the hollow shaft 43.The holes 44 are spaced around the periphery of the forward facingportion 42. Alternatively, the rotating portion 42 can be immersed intoa reservoir of polymer fed by reservoir 80 and pump 81. The rotatingportion 42 then obtains polymer solution from the reservoir and as itrotates in the electrostatic field, a droplet of the solution isaccelerated by the electrostatic field toward the collecting media 70 asdiscussed below.

Facing the emitter 40, but spaced apart therefrom, is a substantiallyplanar grid 60 upon which the collecting media 70, i.e. substrate orcombined substrate is positioned. Air can be drawn through the grid. Thecollecting media 70 is passed around rollers 71 and 72 which arepositioned adjacent opposite ends of grid 60. A high voltageelectrostatic potential is maintained between emitter 40 and grid 60 bymeans of a suitable electrostatic voltage source 61 and connections 62and 63 which connect respectively to the grid 60 and emitter 40.

In use, the polymer solution is pumped to the rotating union 41 orreservoir from reservoir 80. The forward facing portion 42 rotates whileliquid exits from holes 44, or is picked up from a reservoir, and movesfrom the outer edge of the emitter toward collecting media 70 positionedon grid 60. Specifically, the electrostatic potential between grid 60and the emitter 40 imparts a charge to the material which cause liquidto be emitted therefrom as thin fibers which are drawn toward grid 60where they arrive and are collected on substrate 12 or an efficiencylayer 14. In the case of the polymer in solution, solvent is evaporatedoff the fibers during their flight to the grid 60; therefore, the fibersarrive at the substrate 12 or efficiency layer 14. The fine fibers bondto the substrate fibers first encountered at the grid 60. Electrostaticfield strength is selected to ensure that the polymer material as it isaccelerated from the emitter to the collecting media 70, theacceleration is sufficient to render the material into a very thinmicrofiber or nanofiber structure. Increasing or slowing the advancerate of the collecting media can deposit more or less emitted fibers onthe forming media, thereby allowing control of the thickness of eachlayer deposited thereon. The rotating portion 42 can have a variety ofbeneficial positions. The rotating portion 42 can be placed in a planeof rotation such that the plane is perpendicular to the surface of thecollecting media 70 or positioned at any arbitrary angle. The rotatingmedia can be positioned parallel to or slightly offset from parallelorientation.

FIG. 2 is a general schematic diagram of a process and apparatus forforming a layer of fine fiber on a sheet-like substrate or media. InFIG. 2, the sheet-like substrate is unwound at station 20. Thesheet-like substrate 20 a is then directed to a splicing station 21wherein multiple lengths of the substrate can be spliced for continuousoperation. The continuous length of sheet-like substrate is directed toa fine fiber technology station 22 comprising the spinning technology ofFIG. 1 wherein a spinning device forms the fine fiber and lays the finefiber in a filtering layer on the sheet-like substrate. After the finefiber layer is formed on the sheet-like substrate in the formation zone22, the fine fiber layer and substrate are directed to a heat treatmentstation 23 for appropriate processing. The sheet-like substrate and finefiber layer is then tested in an efficiency monitor 24 and nipped ifnecessary at a nip station 25. The sheet-like substrate and fiber layeris then steered to the appropriate winding station to be wound onto theappropriate spindle for further processing 26 and 27.

FIG. 3 is a scanning electron micrograph image showing the relationshipof typical dust particles having a diameter of about 2 and about 5microns with respect to the sizes of pores in typical cellulose mediaand in the typical fine fiber structures. In FIG. 3 a, the 2 micronparticle 31 and the 5 micron particle 32 is shown in a cellulosic media33 with pore sizes that are shown to be quite a bit larger than thetypical particle diameters. In sharp contrast, in FIG. 3B, the 2 micronparticle 31 appears to be approximately equal to or greater than thetypical openings between the fibers in the fiber web 35 while the 5micron particle 32 appears to be larger than any of the openings in thefine fiber web 35.

The foregoing general description of the various aspects of thepolymeric materials of the invention, the fine fiber materials of theinvention including both microfibers and nanofibers and the constructionof useful filter structures from the fine fiber materials of theinvention provides an understanding of the general technologicalprinciples of the operation of the invention. The following specificexemplary materials are examples of materials that can be used in theformation of the fine fiber materials of the invention and the followingmaterials disclose a best mode. The following exemplary materials weremanufactured with the following characteristics and process conditionsin mind. Electrospinning small diameter fiber less than 10 micron isobtained using an electrostatic force from a strong electric fieldacting as a pulling force to stretch a polymer jet into a very finefilament. A polymer melt can be used in the electrospinning process,however, fibers smaller than 1 micron are best made from polymersolution. As the polymer mass is drawn down to smaller diameter, solventevaporates and contributes to the reduction of fiber size. Choice ofsolvent is critical for several reasons. If solvent dries too quickly,then fibers tend to be flat and large in diameter. If the solvent driestoo slowly, solvent will redissolve the formed fibers. Thereforematching drying rate and fiber formation is critical. At high productionrates, large quantities of exhaust air flow helps to prevent a flammableatmosphere, and to reduce the risk of fire. A solvent that is notcombustible is helpful. In a production environment the processingequipment will require occasional cleaning Safe low toxicity solventsminimize worker exposure to hazardous chemicals. Electrostatic spinningcan be done at a flow rate of 1.5 ml/min per emitter, a target distanceof 8 inches, an emitter voltage of 88 kV, an emitter rpm of 200 and arelative humidity of 45%.

The choice of polymer system is important for a given application. Forpulse cleaning application, an extremely thin layer of microfiber canhelp to minimize pressure loss and provide an outer surface for particlecapture and release. A thin layer of fibers of less than 2-microndiameter, preferably less than 0.3-micron diameter is preferred. Goodadhesion between microfiber or nanofiber and substrates upon which themicrofibers or nanofibers are deposited is important. When filters aremade of composites of substrate and thin layer of micro- and nanofibers,such composite makes an excellent filter medium for self-cleaningapplication. Cleaning the surface by back pulsing repeatedly rejuvenatesthe filter medium. As a great force is exerted on the surface, finefiber with poor adhesion to substrates can delaminate upon a back pulsethat passes from the interior of a filter through a substrate to themicro fiber. Therefore, good cohesion between micro fibers and adhesionbetween substrate fibers and electrospun fibers is critical forsuccessful use.

Products that meet the above requirements can be obtained using fibersmade from different polymer materials. Small fibers with good adhesionproperties can be made from such polymers like polyvinylidene chloride,poly vinyl alcohol and polymers and copolymers comprising various nylonssuch as nylon 6, nylon 4,6; nylon 6,6; nylon 6,10 and copolymersthereof. Excellent fibers can be made from PVDF, but to makesufficiently small fiber diameters requires chlorinated solvents. Nylon6, Nylon 66 and Nylon 6,10 can be electrospun. But, solvents such asformic acid, m-cresol, tri-fluoro ethanol, hexafluoro isopropanol areeither difficult to handle or very expensive. Preferred solvents includewater, ethanol, isopropanol, acetone and N-methyl pyrrolidone due totheir low toxicity. Polymers compatible with such solvent systems havebeen extensively evaluated. We have found that fibers made from PVC,PVDC, polystyrene, polyacrylonitrile, PMMA, PVDF require additionaladhesion means to attain structural properties. We also found that whenpolymers are dissolved in water, ethanol, isopropanol, acetone, methanoland mixtures thereof and successfully made into fibers, they haveexcellent adhesion to the substrate, thereby making an excellent filtermedium for self-cleaning application. Self-cleaning via back air pulseor twist is useful when filer medium is used for very high dustconcentration. Fibers from alcohol soluble polyamides and poly(vinylalcohol)s have been used successfully in such applications. Examples ofalcohol soluble polyamides include Macromelt 6238, 6239, and 6900 fromHenkel, Elvamide 8061 and 8063 from duPont and SVP 637 and 651 fromShakespeare Monofilament Company. Another group of alcohol solublepolyamide is type 8 nylon, alkoxy alkyl modifies nylon 66 (Ref. Page447, Nylon Plastics Handbook, Melvin Kohan ed. Hanser Publisher, NewYork, 1995). Examples of poly (vinyl alcohol) include PVA-217, 224 fromKuraray, Japan and Vinol 540 from Air Products and Chemical Company.

We have found that filters can be exposed to extremes in environmentalconditions. Filters in Saudi Arabian desert can be exposed totemperature as high as 150 F.° or higher. Filters installed in Indonesiaor Gulf Coast of US can be exposed high humidity above 90% RH and hightemperature of 100 F.°. Or, they can be exposed to rain. We have foundthat filters used under the hood of mobile equipment like cars, trucks,buses, tractors, and construction equipment can be exposed to hightemperature (+200° F.), high relative humidity and other chemicalenvironment. We have developed test methods to evaluate survivability ofmicrofiber systems under harsh conditions. Soaking the filter mediasamples in hot water (140 F.°) for 5 minutes or exposure to highhumidity, high temperature and air flow.

EXPERIMENTAL

The following materials were produced using the following electrospinprocess conditions.

The following materials were spun using either a rotating emitter systemor a capillary needle system. Both were found to produce substantiallythe same fibrous materials.

The flow rate was 1.5 mil/min per emitter, a target distance of 8inches, an emitter voltage of 88 kV, a relative humidity of 45%, and forthe rotating emitter an rpm of 35.

Example 1 Effect of Fiber Size

Fine fiber samples were prepared from a copolymer of nylon 6, 66, 610nylon copolymer resin (SVP-651) was analyzed for molecular weight by theend group titration. (J. E. Walz and G. B. Taylor, determination of themolecular weight of nylon, Anal. Chem. Vol. 19, Number 7, pp 448-450(1947). Number average molecular weight was between 21,500 and 24,800.The composition was estimated by the phase diagram of melt temperatureof three component nylon, nylon 6 about 45%, nylon 66 about 20% andnylon 610 about 25%. (Page 286, Nylon Plastics Handbook, Melvin Kohaned. Hanser Publisher, New York (1995)). Reported physical properties ofSVP 651 resin are:

Property ASTM Method Units Typical Value Specific Gravity D-792 —   1.08 Water Absorption D-570 %   2.5 (24 hr immersion) Hardness D-240Shore D 65 Melting Point DSC ° C. (° F.) 154 (309) Tensile StrengthD-638 MPa (kpsi)  50 (7.3) @ Yield Elongation at Break D-638 % 350 Flexural Modulus D-790 MPa (kpsi) 180 (26)  Volume Resistivity D-257ohm-cm  10¹²to produce fiber of 0.23 and 0.45 micron in diameter. Samples weresoaked in room temperature water, air-dried and its efficiency wasmeasured. Bigger fiber takes longer time to degrade and the level ofdegradation was less as can be seen in the plot of FIG. 12. Whilewishing not to be limited by certain theory, it appears that smallerfibers with a higher surface/volume ratio are more susceptible todegradation due to environmental effects. However, bigger fibers do notmake as efficient filter medium.

Example 2 Cross-Linking of Nylon Fibers with Phenolic Resin and EpoxyResin

In order to improve chemical resistance of fibers, chemicalcross-linking of nylon fibers was attempted. Copolyamide (nylon 6, 66,610) described earlier is mixed with phenolic resin, identified asGeorgia Pacific 5137 and spun into fiber. Nylon:Phenolic Resin ratio andits melt temperature of blends are shown here;

Composition Melting Temperature (F. °) Polyamide:Phenolic = 100:0 150Polyamide:Phenolic = 80:20 110 Polyamide:Phenolic = 65:35 94Polyamide:Phenolic = 50:50 65

We were able to produce comparable fiber from the blends. The 50:50blend could not be cross-linked via heat as the fibrous structure wasdestroyed. Heating 65:35 blend below 90° C. for 12 hours improves thechemical resistance of the resultant fibers to resist dissolution inalcohol. Blends of polyamide with epoxy resin, such Epon 828 from Shelland Epi-Rez 510 can be used.

Example 3 Surface Modification Though Fluoro Additive (Scotchgard®)Repellant

Alcohol miscible Scotchgard® FC-430 and 431 from 3M Company were addedto polyamide before spinning. Add-on amount was 10% of solids. Additionof Scotchgard® did not hinder fiber formation. THC bench shows thatScotchgard®-like high molecular weight repellant finish did not improvewater resistance. Scotchgard® added samples were heated at 300 F.° for10 minutes as suggested by manufacturer.

Example 4 Modification with Coupling Agents

Polymeric films were cast from polyamides with titanate coupling agentsfrom Kenrich Petrochemicals, Inc. They include isopropyl triisostearoyltitanate (KR TTS), neopentyl (diallyl) oxytri (dioctyl) phosphatotitanate (LICA12), neopentyl (dially) oxy, tri (N-ethylene diamino)ethyl zirconate (NZ44). Cast films were soaked in boiling water. Controlsample without coupling agent loses its strength immediately, whilecoupling agent added samples maintained its form for up to ten minutes.These coupling agents added samples were spun into fiber (0.2 micronfiber).

Example 5 Modification with Low Molecular Weight p-tert-butyl PhenolPolymer

Oligomers of para-tert-butyl phenol, molecular weight range 400 to 1100,was purchased from Enzymol International, Columbus, Ohio. These lowmolecular weight polymers are soluble in low alcohols, such as ethanol,isopropanol and butanol. These polymers were added to co-polyamidedescribed earlier and electrospun into 0.2 micron fibers without adverseconsequences. Some polymers and additives hinder the electrospinningprocess. Unlike the conventional phenolic resin described in Example 2,we have found that this group of polymers does not interfere with fiberforming process.

We have found that this group of additive protects fine fibers from wetenvironment as see in the plot. FIGS. 13-16 show that oligomers providea very good protection at 140 F.°, 100% humidity and the performance isnot very good at 160 F.°. We have added this additive between 5% and 15%of polymer used. We have found that they are equally effectiveprotecting fibers from exposure to high humidity at 140 F.°. We havealso found out that performance is enhanced when the fibers aresubjected to 150 C.° for short period of time.

Table 1 shows the effect of temperature and time exposure of 10% add-onto polyamide fibers.

TABLE 1 Efficiency Retained (%) After 140 deg. F. Soak: Heating TimeTemperature 1 min 3 min 10 min 150 C.° 98.9 98.8 98.5 98.8 98.9 98.8 130C.° 95.4 98.7 99.8 96.7 98.6 99.6 110 C.° 82.8 90.5 91.7 86.2 90.9 85.7

This was a surprising result. We saw dramatic improvement in waterresistance with this family of additives. In order to understand howthis group of additive works, we have analyzed the fine fiber mat withsurface analysis techniques called ESCA. 10% add-on samples shown inTable 1 were analyzed with ESCA at the University of Minnesota with theresults shown in Table 2.

TABLE 2 Surface Composition (Polymer:Additive Ratio) Heating TimeTemperature 1 min 3 min 10 min 150 C.°. 40:60 40:60 50:50 130 C.°. 60:4056:44 62:82 110 C.°. 63:37 64:36 59:41 No Heat 77:23

Initially, it did not seem to make sense to find surface concentrationof additive more than twice of bulk concentration. However, we believethat this can be explained by the molecular weight of the additives.Molecular weight of the additive of about 600 is much smaller than thatof host fiber forming polymer. As they are smaller in size, they canmove along evaporating solvent molecules. Thus, we achieve highersurface concentration of additives. Further treatment increases thesurface concentration of the protective additive. However, at 10 minexposure, 150 C.°, did not increase concentration. This may be anindication that mixing of two components of copolyamide and oligomermolecules is happening as long chain polymer has a time to move around.What this analysis has taught us is that proper selection of posttreatment time and temperature can enhance performance, while too longexposure could have a negative influence.

We further examined the surface of these additive laden microfibersusing techniques called Time of Flight SIMS. This technique involvesbombarding the subject with electrons and observes what is coming fromthe surface. The samples without additives show organic nitrogen speciesare coming off upon bombardment with electron. This is an indicationthat polyamide species are broken off. It also shows presence of smallquantity of impurities, such as sodium and silicone. Samples withadditive without heat treatment (23% additive concentration on surface)show a dominant species of t-butyl fragment, and small but unambiguouspeaks observed peaks observed for the polyamides. Also observed are highmass peaks with mass differences of 148 amu, corresponding to t-butylphenol. For the sample treated at 10 min at 150 C.° (50% surfaceadditive concentration by ESCA analysis), inspection shows dominance oft-butyl fragments and trace, if at all, of peaks for polyamide. It doesnot show peaks associated with whole t-butyl phenol and its polymers. Italso shows a peak associated with C₂H₃O fragments.

The ToF SIMS analysis shows us that bare polyamide fibers will give offbroken nitrogen fragment from exposed polymer chain and contaminants onthe surface with ion bombardment. Additive without heat treatment showsincomplete coverage, indicating that additives do not cover portions ofsurface. The t-butyl oligomers are loosely organized on the surface.When ion beam hits the surface, whole molecules can come off along withlabile t-butyl fragment. Additive with heat treatment promotes completecoverage on the surface. In addition, the molecules are tightly arrangedso that only labile fragments such as t-butyl-, and possibly CH═CH—OH,are coming off and the whole molecules of t-butyl phenol are not comingoff. ESCA and ToF SIMS look at different depths of surface. ESCA looksat deeper surface up to 100 Angstrom while ToF SIMS only looks at10-Angstrom depth. These analyses agree.

Example 6 Development of Surface Coated Interpolymer

Type 8 Nylon was originally developed to prepare soluble andcrosslinkable resin for coating and adhesive application. This type ofpolymer is made by the reaction of polyamide 66 with formaldehyde andalcohol in the presence of acid. (Ref Cairns, T. L.; Foster, H. D.;Larcher, A. W.; Schneider, A. K.; Schreiber, R. S. J. Am. Chem. Soc.1949, 71, 651). This type of polymer can be electrospun and can becrosslinked. However, formation of fiber from this polymer is inferiorto copolyamides and crosslinking can be tricky.

In order to prepare type 8 nylon, 10-gallon high-pressure reactor wascharged with the following ratio:

Nylon 66 (duPont Zytel 101)   10 pounds Methanol 15.1 pounds Water  2.0pounds Formaldehyde 12.0 pounds

The reactor is then flushed with nitrogen and is heated to at least 135C.° under pressure. When the desired temperature was reached, smallquantity of acid was added as catalyst. Acidic catalysts includetrifluoroacetic acid, formic acid, toluene sulfonic acid, maleic acid,maleic anhydride, phthalic acid, phthalic anhydride, phosphoric acid,citric acid and mixtures thereof. Nafion® polymer can also be used as acatalyst. After addition of catalyst, reaction proceeds up to 30minutes. Viscous homogeneous polymer solution is formed at this stage.After the specified reaction time, the content of the high pressurevessel is transferred to a bath containing methanol, water and base,like ammonium hydroxide or sodium hydroxide to shortstop the reaction.After the solution is sufficiently quenched, the solution isprecipitated in deionized water. Fluffy granules of polymer are formed.Polymer granules are then centrifuged and vacuum dried. This polymer issoluble in, methanol, ethanol, propanol, butanol and their mixtures withwater of varying proportion. They are also soluble in blends ofdifferent alcohols.

Thus formed alkoxy alkyl modified type 8 polyamide is dissolved inethanol/water mixture. Polymer solution is electrospun in a mannerdescribed in Barris et al., U.S. Pat. No. 4,650,506. Polymer solutionviscosity tends to increase with time. It is generally known thatpolymer viscosity has a great influence in determining fiber sizes.Thus, it is difficult to control the process in commercial scale,continuous production. Furthermore, under same conditions, type 8polyamides do not form microfibers as efficiently as copolyamides.However, when the solution is prepared with addition of acidic catalyst,such as toluene sulfonic acid, maleic anhydride, trifluoro methanesulfonic acid, citric acid, ascorbic acid and the like, and fiber matsare carefully heat-treated after fiber formation, the resultant fiberhas a very good chemical resistance. (FIG. 13). Care must be takenduring the crosslinking stage, so that one does not destroy fibrousstructure.

We have found a surprising result when type 8 polyamide (polymer B) isblended with alcohol soluble copolyamides. By replacing 30% by weight ofalkoxy alkyl modified polyamide 66 with alcohol soluble copolyamide likeSVP 637 or 651 (polymer A), Elvamide 8061, synergistic effects werefound. Fiber formation of the blend is more efficient than either of thecomponents alone. Soaking in ethanol and measuring filtration efficiencyshows better than 98% filtration efficiency retention. THC bench testingshowing comparable results with Type 8 polyamide alone. This type blendshows that we can obtain advantage of efficient fiber formation andexcellent filtration characteristic of copolyamide with advantage ofexcellent chemical resistance of crosslinked type 8 polyamide. Alcoholsoak test strongly suggests that non-crosslinkable copolyamide hasparticipated in crosslinking to maintain 98% of filtration efficiency.

DSC (see FIGS. 17-20) of blends of polymer A and B becomeindistinguishable from that of polymer A alone after they are heated to250 C.° (fully crosslinked) with no distinct melt temperature. Thisstrongly suggests that blends of polymer A and B are a fully integratedpolymer by polymer B crosslinking with polymer A. This is a completelynew class of polyamide.

Similarly, melt-blend poly (ethylene terephthalate) with poly(butyleneterephthalate) can have similar properties. During the melt processingat temperatures higher than melt temperature of either component, estergroup exchange occurs and inter polymer of PET and PBT formed.Furthermore, our crosslinking temperature is lower than either of singlecomponent. One would not have expected that such group exchange occur atthis low temperature. Therefore, we believe that we found a new familyof polyamide through solution blending of Type A and Type B polyamideand crosslinking at temperature lower than the melting point of eithercomponent.

When we added 10% by weight of t-butyl phenol oligomer (Additive 7) andheat treated at temperature necessary for crosslinking temperature, wehave found even better results. We theorized that hydroxyl functionalgroup of t-butyl phenol oligomers would participate in reaction withfunctional group of type 8 nylons. What we have found is this componentsystem provides good fiber formation, improved resistance to hightemperature and high humidity and hydrophobicity to the surface of finefiber layers.

We have prepared samples of mixture of Polymer A and Polymer B (Sample6A) and another sample of mixture of Polymer A, Polymer B and Additive &(Sample 6B). We then formed fiber by electrospinning process, exposedthe fiber mat at 300° F. for 10 minutes and evaluated the surfacecomposition by ESCA surface analysis.

TABLE 3 ESCA analysis of Samples 6A and 6B. Composition (%) Sample 6ASample 6B Polymer A 30 30 Polymer B 70 70 Additive 7 0 10 SurfaceComposition W/O Heat W/Heat W/O Heat W/Heat Polymer A&B (%) 100 100 68.943.0 Additive 7 0 0 31.1 57.0

ESCA provides information regarding surface composition, except theconcentration of hydrogen. It provides information on carbon, nitrogenand oxygen. Since the Additive 7 does not contain nitrogen, we canestimate the ratio of nitrogen containing polyamides and additive thatdoes not contain nitrogen by comparing concentration of nitrogen.Additional qualitative information is available by examining O 1sspectrum of binding energy between 535 and 527 eV. C═O bond has abinding energy at around 531 eV and C—O bond has a binding energy at 533eV. By comparing peak heights at these two peaks, one can estimaterelative concentration of polyamide with predominant C═O and additivewith solely C—O groups. Polymer B has C—O linkage due to modificationand upon crosslinking the concentration of C—O will decrease. ESCAconfirms such reaction had indeed occurred, showing relative decrease ofC—O linkage. (FIG. 4 for non heat treated mixture fiber of Polymer A andPolymer B, FIG. 5 for heat treated mixture fiber of Polymer A andPolymer B). When Additive 7 molecules are present on the surface, onecan expect more of C—O linkage. This is indeed the case as can be seenin FIGS. 6 and 7. (FIG. 6 for as-spun mixture fibers of Polymer A,Polymer B and Additive 7. FIG. 7 for heat treated mixture fibers ofPolymer A, Polymer B and Additive 7). FIG. 6 shows that theconcentration of C—O linkage increases for Example 7. The finding isconsistent with the surface concentration based on XPS multiplexspectrum of FIGS. 8 through 11.

It is apparent that t-butyl oligomer molecules migrated toward thesurface of the fine fibers and form hydrophobic coating of about 50 Å.Type 8 nylon has functional groups such as —CH₂OH and —CH₂OCH₃, which weexpected to react with —OH group of t-butyl phenol. Thus, we expected tosee less oligomer molecules on the surface of the fibers. We have foundthat our hypothesis was not correct and we found the surface of theinterpolymer has a thin coating.

Samples 6A, 6B and a repeat of sample described in Section 5 have beenexposed THC bench at 160° F. at 100% RH. In previous section, thesamples were exposed to 140° F. and 100% RH. Under these conditions,t-butyl phenol protected terpolymer copolyamide from degradation.However, if the temperature is raised to 160° F. and 100% RH, then thet-butyl phenol oligomer is not as good in protecting the underlyingterpolymer copolyamide fibers. We have compared samples at 160° F. and100% RH.

TABLE 4 Retained Fine Fiber Efficiency after Exposure to 160° F. and100% RH Sample After 1 Hr. After 2 Hrs. After 3 Hrs. Sample 6A 82.6 82.685.9 Sample 6B 82.4 88.4 91.6 Sample 5 10.1The table shows that Sample 6B helps protect exposure to hightemperature and high humidity.

More striking difference shows when we exposed to droplets of water on afiber mat. When we place a drop of DI water in the surface of Sample 6A,the water drops immediately spread across the fiber mat and they wet thesubstrate paper as well. On the other hand, when we place a drop ofwater on the surface of Sample 6B, the water drop forms a bead and didnot spread on the surface of the mat. We have modified the surface ofSample 16 to be hydrophobic by addition of oligomers of p-t-butylphenol. This type of product can be used as a water mist eliminator, aswater drops will not go through the fine fiber surface layer of Sample6B.

Samples 6A, 6B and a repeat sample of Section 5 were placed in an ovenwhere the temperature was set at 310° F. Table shows that both Samples6A and 6B remain intact while Sample of Section 5 was severely damaged.

TABLE 5 Retained Fine Fiber Efficiency after Exposure to 310° F. SampleAfter 6 Hrs. After 77 Hrs. Sample 6A 100% 100% Sample 6B 100% 100%Sample 5 34% 33%

While addition of oligomer to Polymer A alone improved the hightemperature resistance of fine fiber layer, the addition of Additive 7has a neutral effect on the high temperature exposure.

We have clearly shown that the mixture of terpolymer copolyamide, alkoxyalkyl modified nylon 66 and oligomers of t-butyl phenol provides asuperior products in helping fine fibers under severe environment withimproved productivity in manufacturing over either mixture of terpolymercopolyamide and t-butyl phenol oligomer or the mixture of terpolymercopolyamide and alkoxy alkyl modified nylon 66. These two componentsmixture are also improvement over single component system.

Example 7 Compatible Blend of Polyamides and Bisphenol A polymers

A new family of polymers can be prepared by oxidative coupling ofphenolic ring (Pecora, A; Cyrus, W. U.S. Pat. No. 4,900,671 (1990) andPecora, A; Cyrus, W.; Johnson, M. U.S. Pat. No. 5,153,298 (1992)). Ofparticular interest is polymer made of Bisphenol A sold by Enzymol Corp.Soybean Peroxidase catalyzed oxidation of Bisphenol A can start fromeither side of two —OH groups in Bisphenol A. Unlike Bisphenol A basedpolycarbonate, which is linear, this type of Bisphenol A polymer formshyperbranched polymers. Because of hyperbranched nature of this polymer,they can lower viscosity of polymer blend.

We have found that this type of Bisphenol A polymer can be solutionblended with polyamides. Reported Hansen's solubility parameter fornylon is 18.6. (Page 317, Handbook of Solubility Parameters and othercohesion parameters, A. Barton ed., CRC Press, Boca Raton Fla., 1985).If one calculates solubility parameter (page 61, Handbook of SolubilityParameters), then the calculated solubility parameter is 28.0. Due tothe differences in solubility parameter, one would not expect that theywould be miscible with each other. However, we found that they are quitemiscible and provide unexpected properties.

50:50 blend of Bisphenol A resin of M.W. 3,000 and copolyamide was madein ethanol solution. Total concentration in solution was 10%.Copolyamide alone would have resulted in 0.2 micron fiber diameter.Blend resulted in lofty layer of fibers around 1 micron. Bisphenol A of7,000 M.W. is not stable with copolyamide and tends to precipitate.

DSC of 50:50 blend shows lack of melting temperature. Copolyamide hasmelting temperature around 150° C. and Bisphenol A resin is a glassypolymer with Tg of about 100. The blend shows lack of distinct melting.When the fiber mat is exposed to 100° C., the fiber mat disappears. Thisblend would make an excellent filter media where upper use temperatureis not very high, but low-pressure drop is required. This polymer systemcould not be crosslinked with a reasonable manner.

Example 8 Dual Roles of Bisphenol A Polymer As Solvent and Solid inBlend

A surprising feature of Bisphenol A polymer blend is that in solutionform Bisphenol A polymer acts like a solvent and in solid form thepolymer acts as a solid. We find dual role of Bisphenol A polymer trulyunique.

The following formulation is made:

Alkoxy alkyl modified PA 66: Polymer B 180 g Bisphenol A Resin (3,000MW): Polymer C 108 g Ethanol 190 Grade 827 g Acetone 218 8 DI water 167g Catalyst  9.3 g

The viscosity of this blend was 32.6 centipoise by Brookfieldviscometer. Total polymer concentration was be 19.2%. Viscosity ofPolymer B at 19.2% is over 200 centipoise. Viscosity of 12% polymer Balone in similar solvent is around 60 centipoise. This is a clearexample that Bisphenol A resin acts like a solvent because the viscosityof the total solution was lower than expected. Resultant fiber diameterwas 0.157 micron. If polymer B alone participated in fiber formation,the expected fiber size would be less than 0.1 micron. In other words,Polymer C participated in fiber formation. We do not know of any othercase of such dramatic dual role of a component. After soaking the samplein ethanol, the filtration efficiency and fiber size was measured. Afteralcohol soak, 85.6% of filtration efficiency was retained and the fibersize was unchanged. This indicates that Polymer C has participated incrosslinking acting like a polymer solid.

Another polymer solution was prepared in the following manner:

Alkoxy alkyl Modified PA66: Polymer B 225 g Bisphenol A Resin (3,000MW): Polymer C 135 g Ethanol 190 Grade 778 g Acetone 205 g DI Water 157g Catalyst 11.6 g 

Viscosity of this blend was 90.2 centipoise. This is a very lowviscosity value for 24% solid. Again, this is an indication Polymer Cacts like a solvent in the solution. However, when they are electrospuninto fiber, the fiber diameter is 0.438 micron. 15% solution of PolymerB alone would have produced around 0.2-micron fibers. In final state,Polymer C contributes to enlarging fiber sizes. Again, this exampleillustrates that this type of branched polymer acts as a solvent insolution and acts as a solid in final state. After soaking in ethanolsolution, 77.9% of filtration efficiency was retained and fiber size wasunchanged.

Example 9 Development of Crosslinked Polyamides/Bisphenol A PolymerBlends

Three different samples were prepared by combining resins, alcohols andwater, stirring 2 hours at 60° C. The solution is cooled to roomtemperature and catalyst was added to solution and the mixture wasstirred another 15 minutes. Afterward, viscosity of solution wasmeasured and spun into fibers.

-   -   The following table shows these examples:

Recipe (g) Sample 9A Sample 9B Sample 9C Polymer B 8.4 12.6 14.7 PolymerA 3.6 5.4 6.3 Polymer C 7.2 10.8 12.6 Ethanol 190 Grade 89.3 82.7 79.5Isopropanol 23.5 21.8 21.0 DI Water 18.0 16.7 15.9 Catalyst .45 0.580.79 Viscosity (cP) 22.5 73.5 134.2 Fiber Size (micron) 0.14 0.258 0.496

We have found out that this blend generates fibers efficiently,producing about 50% more mass of fiber compared to Polymer A recipe. Inaddition, resultant polymeric microfibers produce a more chemicallyresistant fiber. After alcohol soak, a filter made from these fibersmaintained more than 90% filtration efficiency and unchanged fiberdiameter even though inherently crosslinkable polymer is only 44% of thesolid composition. This three-polymer composition of co-polyamide,alkoxy alkyl modified Nylon 66 and Bisphenol A creates excellent fiberforming, chemically resistant material.

Example 10 Alkoxy Alkyl Modified Co-Polymer of Nylon 66 and Nylon 46

In a 10-gallon high-pressure reactor, the following reactions were made,and resultant polymers were analyzed. After reaction temperature wasreached, catalyst were added and reacted for 15 minutes. Afterward, thepolymer solution was quenched, precipitated, washed and dried.

Run Run Run Run Reactor Charge (LB) 10A 10B 10C 10D Run 10E Nylon 4,6(duPont 10 5 5 5 5 Zytel 101) Nylon 6,6 (DSM 0 5 5 5 5 Stanyl 300)Formaldehyde 8 10 8 10 8 DI Water 0.2 0.2 2 0.2 2 Methanol 22 20 20 2020 Reaction Temp (C.°) 140 140 140 150 150 Tg (C.°) 56.7 38.8 37.7 38.531.8 Tm (C.°) 241.1 162.3 184.9 175.4 189.5 Level of Substitution Alkoxy(wt. %) 11.9 11.7 7.1 11.1 8.4 Methylol (wt %) 0.14 0.13 0.14 0.26 0.24

DSC of the polymer made with Nylon 46 and Nylon 66 shows broad singlemelt temperature, which is lower than the melting temperature ofmodified Nylon 46 (241) C.° or modified Nylon 66 (210 C.°). This is anindication that during the reaction, both components are randomlydistributed along the polymer chain. Thus, we believe that we haveachieved random copolymer of Nylon 46 and Nylon 66 with alkoxy alkylmodification. These polymers are soluble in alcohols and mixtures ofalcohol and water.

Property ASTM Nylon 6.6 Nylon 4.6 T_(m) 265° C. 295° C. Tensile StrengthD638 13.700 8.500 Elongation at Break D638 15-80   60 Tensile YieldStrength D638 8000-12,000 Flexural Strength D790 17,8000 11,500 TensileModulus × 10³ psi D638 230-550   250 Izod Impact ft-lb/in of notch D256A0.55-1.0   17 Deflection Temp Under D648 158 194 Flexural Load 264 psiBoth are highly crystalline and are not soluble in common alcohols.Source: Modern Plastics Encyclopedia 1998

Example 11 Development of Interpolymer of Coployamides and AlkoxyalkylModified Nylon 46/66 Copolymer and Formation of Electrospun Fibers

Runs 10B and 10D samples were made into fibers by methods described inabove. Alkoxy alkyl modified Nylon 46/66 (Polymer D) alone weresuccessfully electrospun. Blending Polymer D with Polymer A bringsadditional benefits of more efficient fiber formation and ability tomake bigger fibers without sacrificing the crosslinkability of Polymer Das can be seen in the following table:

Polymer 10B Polymer 10D w/30% w/30% Alone Polymer A Alone Polymer AFiber Size (micron) 0.183 0.464 0.19 0.3 Fiber Mass Ratio 1 3 1 2Filtration Effi. 87 90 92 90 Retention (%)Fiber Mass Ratio is calculated by (total length of fiber times crosssectional area). Filtration Efficiency Retention is measured soakingfilter sample in ethanol. Fiber size was unchanged by alcohol soak.

Example 12 Crosslinked, Electrospun PVA

PVA powders were purchased from Aldrich Chemicals. They were dissolvedeither in water or 50/50 mixture of methanol and water. They were mixedwith crosslinking agent and toluene sulfonic acid catalyst beforeelectrospinning. The resulting fiber mat was crosslinked in an oven at150° C. for 10 minutes before exposing to THC bench.

Sample 12A Sample 12B Sample 12C Sample 12D PVA Hydrolysis 98-99 87-8987-89 87-89 M.W. 31,500- 31,500- 31,500- 31,500- 50,000 50,000 50,00050,000 PVA 10 10 10 10 Conc. (%) Solvent Water Mixture Mixture(c)Mixture(d) Other Polymer None None Acrylic Acid Cymel 385 Other Polymer/ 0  0 30 30 PVA (%) % Fiber 0 (a) 0 (a, b) 95 (b) 20 (b) Retained THC, 1hr. % Fiber 90 (a) Retained THC, 3 hr. (a): Temperature 160° F., 100%humidity (b): Temperature 140° F., 100% humidity (c): Molecular Weight2000 (d): Melamine formaldehyde resin from Cytec

Example 13

A conventional cellulose air filter media was used as the substrate.This substrate had a basis weight of 67 pounds per 3000 square feet, aFrazier permeability of 16 feet per minute at 0.5 inch of water pressuredrop, a thickness of 0.012 inch, and a LEFS efficiency of 41.6%. A finefiber layer of Example 1 was added to the surface using the processdescribed with a nominal fiber diameter of 0.2 micron. The resultingcomposite had a LEFS efficiency of 63.7%. After exposure to 140 F air at100% relative humidity for 1 hour the substrate only sample was allowedto cool and dry, it then had a LEFS efficiency of 36.5%. After exposureto 140 F air at 100% relative humidity for 1 hour the composite samplewas allowed to cool and dry, it then had a LEFS efficiency of 39.7%.Using the mathematical formulas described, the fine fiber layerefficiency retained after 1 hour of exposure was 13%, the number ofeffective fine fibers retained was 11%.

Example 14

A conventional cellulose air filter media was used as the substrate.This substrate had a basis weight of 67 pounds per 3000 square feet, aFrazier permeability of 16 feet per minute at 0.5 inch of water pressuredrop, a thickness of 0.012 inch, and a LEFS efficiency of 41.6%. A finefiber layer of Example 5 was added to the surface using the processdescribed with a nominal fiber diameter of 0.2 micron. The resultingcomposite had a LEFS efficiency of 96.0%. After exposure to 160° F. airat 100% relative humidity for 3 hours the substrate only sample wasallowed to cool and dry, it then had a LEFS efficiency of 35.3%. Afterexposure to 160° F. air at 100% relative humidity for 3 hours thecomposite sample was allowed to cool and dry, it then had a LEFSefficiency of 68.0%. Using the mathematical formulas described, the finefiber layer efficiency retained after 3 hours of exposure was 58%, thenumber of effective fine fibers retained was 29%.

Example 15

A conventional cellulose air filter media was used as the substrate.This substrate had a basis weight of 67 pounds per 3000 square feet, aFrazier permeability of 16 feet per minute at 0.5 inch of water pressuredrop, a thickness of 0.012 inch, and a LEFS efficiency of 41.6%. A finefiber layer of a blend of Polymer A and Polymer B as described inExample 6 was added to the surface using the process described with anominal fiber diameter of 0.2 micron. The resulting composite had a LEFSefficiency of 92.9%. After exposure to 160° F. air at 100% relativehumidity for 3 hours the substrate only sample was allowed to cool anddry, it then had a LEFS efficiency of 35.3%. After exposure to 160° F.air at 100% relative humidity for 3 hours the composite sample wasallowed to cool and dry, it then had a LEFS efficiency of 86.0%. Usingthe mathematical formulas described, the fine fiber layer efficiencyretained after 3 hours of exposure was 96%, the number of effective finefibers retained was 89%.

Example 16

A conventional cellulose air filter media was used as the substrate.This substrate had a basis weight of 67 pounds per 3000 square feet, aFrazier permeability of 16 feet per minute at 0.5 inch of water pressuredrop, a thickness of 0.012 inch, and a LEFS efficiency of 41.6%. A finefiber layer of Polymer A, Polymer B, t-butyl phenol oligomer asdescribed in Example 6 was added to the surface using the processdescribed with a nominal fiber diameter of 0.2 micron. The resultingcomposite had a LEFS efficiency of 90.4%. After exposure to 160° F. airat 100% relative humidity for 3 hours the substrate only sample wasallowed to cool and dry, it then had a LEFS efficiency of 35.3%. Afterexposure to 160° F. air at 100% relative humidity for 3 hours thecomposite sample was allowed to cool and dry, it then had a LEFSefficiency of 87.3%. Using the mathematical formulas described, the finefiber layer efficiency retained after 3 hours of exposure was 97%, thenumber of effective fine fibers retained was 92%.

Example 17

A conventional cellulose air filter media was used as the substrate.This substrate had a basis weight of 67 pounds per 3000 square feet, aFrazier permeability of 16 feet per minute at 0.5 inch of water pressuredrop, a thickness of 0.012 inch, and a LEFS efficiency of 41.6%. A finefiber layer of crosslinked PVA with polyacrylic acid of Example 12 wasadded to the surface using the process described with a nominal fiberdiameter of 0.2 micron. The resulting composite had a LEFS efficiency of92.9%. After exposure to 160° F. air at 100% relative humidity for 2hours the substrate only sample was allowed to cool and dry, it then hada LEFS efficiency of 35.3%. After exposure to 160° F. air at 100%relative humidity for 2 hours the composite sample was allowed to cooland dry, it then had a LEFS efficiency of 83.1%. Using the mathematicalformulas described, the fine fiber layer efficiency retained after 2hours of exposure was 89%, the number of effective fine fibers retainedwas 76%.

Example 18

The following filter medias have been made with the methods described inExamples 1-17.

Filter Media Examples

Substrate Substrate Basis wt Substrate Substrate Composite perm(lbs/3000 Thickness Eff Eff Substrate (Frazier) sq ft) (in) (LEFS)(LEFS) Single fine fiber (+/−10%) (+/−10%) (+/−25%) (+/−5%) (+/−5%)layer on single substrate (flow either direction through media Celluloseair 58 67 0.012 11% 50% filter media Cellulose air 16 67 0.012 43% 58%filter media Cellulose air 58 67 0.012 11% 65% filter media Celluloseair 16 67 0.012 43% 70% filter media Cellulose air 22 52 0.010 17% 70%filter media Cellulose air 16 67 0.012 43% 72% filter media Cellulose/14 70 0.012 30% 70% synthetic blend with moisture resistant resin Flameretardant 17 77 0.012 31% 58% cellulose air filter media Flame retardant17 77 0.012 31% 72% cellulose air filter media Flame retardant 27 830.012 77% synthetic air filter media Spunbond 1200 15 0.007 5% 55% Remay(polyester) Synthetic/ 260 76 0.015 6% 17% cellulose air filter mediaSynthetic/glass 31 70 0.012 55% 77% air filter media Synthetic/glass 3170 0.012 50% 90% air filter media Synthetic 300 25 0.008 3% 65%(Lutrador- polyester) Synthetic 0.016 90% (Lutrador- polyester)

Media has been used flat, corrugated, pleated, corrugated and pleated,in flatsheets, pleated flat panels, pleated round filters, and Zfilters.

Test Methods

Hot Water Soak Test

Using filtration efficiency as the measure of the number of fine fiberseffectively and functionally retained in structure has a number ofadvantages over other possible methods such as SEM evaluation.

-   -   the filtration measure evaluates several square inches of media        yielding a better average than the tiny area seen in SEM        photomicrographs (usually less than 0.0001 square inch)    -   the filtration measurement quantifies the number of fibers        remaining functional in the structure. Those fibers that remain,        but are clumped together or otherwise existing in an altered        structure are only included by their measured effectiveness and        functionality.

Nevertheless, in fibrous structures where the filtration efficiency isnot easily measured, other methods can be used to measure the percent offiber remaining and evaluated against the 50% retention criteria.

Description: This test is an accelerated indicator of filter mediamoisture resistance. The test uses the LEFS test bench to measure filtermedia performance changes upon immersion in water. Water temperature isa critical parameter and is chosen based on the survivability history ofthe media under investigation, the desire to minimize the test time andthe ability of the test to discriminate between media types. Typicalwater temperatures are 70° F., 140° F. or 160° F.

Procedure:

A 4″ diameter sample is cut from the media. Particle capture efficiencyof the test specimen is calculated using 0.8™ latex spheres as a testchallenge contaminant in the LEFS (for a description of the LEFS test,see ASTM Standard F1215-89) bench operating at 20 FPM. The sample isthen submerged in (typically 140° F.) distilled water for 5 minutes. Thesample is then placed on a drying rack and dried at room temperature(typically overnight). Once it is dry the sample is then retested forefficiency on the LEFS bench using the same conditions for the initialcalculation.

The previous steps are repeated for the fine fiber supporting substratewithout fine fiber.

From the above information one can calculate the efficiency componentdue only to the fine fiber and the resulting loss in efficiency due towater damage. Once the loss in efficiency due to the fine fiber isdetermined one can calculate the amount of efficiency retained.

Calculations:

Fine fiber layer efficiency: E_(i)=Initial Composite Efficiency;

-   -   E_(S)=Initial Substrate Efficiency;    -   F_(e)=Fine Fiber Layer    -   F_(e)=1−EXP(Ln(1−E₁)−Ln(1−E_(x)))        Fine fiber layer efficiency retained: F_(i)=Initial fine fiber        layer efficiency;    -   F_(x)=Post soak fine fiber layer efficiency;    -   F_(r)=Fine fiber retained        F _(r) ═F _(x) /F _(i)        The percentage of the fine fibers retained with effective        functionality can also be calculated by:        %=log(1−F _(x))/log(1−F _(i))        Pass/Fail Criteria: >50% efficiency retention

In most industrial pulse cleaning filter applications the filter wouldperform adequately if at least 50% of the fine fiber efficiency isretained.

THC Bench (Temperature, Humidity

Description: The purpose of this bench is to evaluate fine fiber mediaresistance to the affects of elevated temperature and high humidityunder dynamic flow conditions. The test is intended to simulate extremeoperating conditions of either an industrial filtration application, gasturbine inlet application, or heavy duty engine air intake environments.Samples are taken out, dried and LEFS tested at intervals. This systemis mostly used to simulate hot humid conditions but can also be used tosimulate hot/cold dry situations.

Temperature −31 to 390° F. Humidity 0 to 100% RH (Max temp for 100% RHis 160° F. and max continuous duration at this condition is 16 hours)Flow Rate 1 to 35 FPMProcedure:

A 4″ diameter sample is cut from the media.

Particle capture efficiency of the test specimen is calculated using 0.8Tm latex spheres as a test challenge contaminant in the LEFS benchoperating at 20 FPM.

The sample is then inserted into the THC media chuck.

Test times can be from minutes to days depending on testing conditions.

The sample is then placed on a drying rack and dried at room temperature(typically overnight). Once it is dry the sample is then retested forefficiency on the LEFS bench using the same conditions for the initialcalculation.

The previous steps are repeated for the fine fiber supporting substratewithout fine fiber. From the above information one can calculate theefficiency component due only to the fine fiber and the resulting lossin efficiency due to alcohol damage.

Once the loss in efficiency due to the fine fiber is determined one cancalculate the amount of efficiency retained.

Pass/Fail Criteria: >50% efficiency retention

In most industrial pulse cleaning filter applications the filter wouldperform adequately if at least 50% of the fine fiber efficiency isretained.

Alcohol (Ethanol) Soak Test

Description: The test uses the LEFS test bench to measure filter mediaperformance changes upon immersion in room temperature ethanol.

Procedure:

A 4″ diameter sample is cut from the media. Particle capture efficiencyof the test specimen is calculated using 0.8 Tm latex spheres as a testchallenge contaminant in the LEFS bench operating at 20 FPM. The sampleis then submerged in alcohol for 1 minute.

The sample is then placed on a drying rack and dried at room temperature(typically overnight). Once it is dry the sample is then retested forefficiency on the LEFS bench using the same conditions for the initialcalculation. The previous steps are repeated for the fine fibersupporting substrate without fine fiber. From the above information onecan calculate the efficiency component due only to the fine fiber andthe resulting loss in efficiency due to alcohol damage. Once the loss inefficiency due to the fine fiber is determined one can calculate theamount of efficiency retained.

Pass/Fail Criteria: >50% efficiency retention.

The above specification, examples and data provide an explanation of theinvention. However, many variations and embodiments can be made to thedisclosed invention. The invention is embodied in the claims hereinafter appended.

We claim:
 1. A filter media comprising a fine fiber layer and afiltration substrate layer, the substrate layer comprising fibercomprising a cellulosic fiber or a synthetic polymer, the substratehaving a thickness of 25 to 800 microns and a basis weight of about 8 to200 gm-m⁻², the fine fiber layer having a thickness no greater than 30microns and a basis weight of about 0.0001 to 2.4 g-m⁻², the fine fibercomprising a fiber size of about 0.01 to 2 micron.
 2. The media of claim1 wherein the fine fiber comprises a polymeric core and an oligomericcoating.
 3. The media of claim 2 wherein the coating comprises ahydrophobic additive.
 4. The media of claim 1 wherein the fine fiber hasa diameter of 0.01 to 0.5 microns.
 5. The media of claim 1 wherein thefine fiber has a diameter of 0.1 to 0.2 microns.
 6. The media of claim 1wherein the substrate comprises a spunbonded fabric.
 7. The media ofclaim 1 wherein the substrate comprises a woven or non-woven fabrichaving a basis weight of 0.5 to 150 gm-m⁻².
 8. The media of claim 1wherein the substrate comprises a polyolefin.
 9. The media of claim 8wherein the substrate comprises a polyethylene.
 10. The media of claim 8wherein the substrate comprises a polypropylene.
 11. The media of claim6 wherein the spunbonded fabric comprises polyolefin spunbonded fabric.12. The media of claim 11 wherein the polyolefin spunbonded fabriccomprises polypropylene spunbonded fabric.